Add implementations for modify options for `EtcdFdw`

[if-loop69420]

In order to support writes we need to implement the rest of the functions for ForeignDataWrapper have to be implemented.
These are as follows:

• begin_modify(...) -> Result<(), EtcdFdwError>
• insert(&mut self, _row: &Row) -> Result<(), EtcdFdwError>
• update(&mut self, _rowid: &Cell, _new_row: &Row) -> Result<(), EtcdFdwError>
• delete(&mut self, _rowid: &Cell) -> Result<(), EtcdFdwError>
• get_rel_size(...) -> Result<(i64, i32), EtcdFdwError>
• end_modify(&mut self) -> Result<(), EtcdFdwError>